An absolute encoder provides a readout of the position of an apparatus relative to some predetermined location. For example, an absolute shaft encoder provides a readout of the number of degrees the shaft would need to be rotated to return to a predetermined starting position.
Conventional absolute encoders utilize a series of fiducial marks and detectors to provide the above-described readout. In general, if the device provides an N-bit readout of the location, there are N separate sets of fiducial marks, one per bit. The marks for each set are arranged as a “track”. There are also N separate detectors, one per track. The fiducial marks are normally placed on the moving apparatus, and the detectors are placed on a device that is fixed relative to the moving apparatus such that each set of fiducial marks moves past the corresponding detector as the apparatus moves. Each detector provides a signal when one of the associated fiducial marks passes the detector. However, systems in which the detectors are on the moving apparatus and the fiducials are on the fixed surface are also known.
This type of arrangement has severe alignment requirements that substantially increase the cost of encoders based on this type of design. In particular, the individual sets of fiducial marks must be aligned relative to one another. Similarly, detectors must also be aligned relative to one another. The alignment tolerance is determined by the smallest distance that is to be resolved. Hence, systems in which N is large are particularly costly both in terms of the number of encoding tracks and detectors and in terms of the alignment costs.